The issues I have with preparations have more to do with timing than successes - preparations last for short enough that you need to make them right before they're used, but take quite a while to make. You can't bring many preparations on a run, since the time to make multiple will often be enough for the first few to run out. You can't deal with unexpected delays, like learning that the ideal time for a run is the next day after you already made your preparation.
Any investment in Enchanting for the sake of preparations has to deal with the fact that you make an average of something like 1 enchanting roll per session. And that roll will then have to be rolled again before it actually takes effect.
I could probably be convinced that low-investment enchanting is a good idea - someone with magic 6, Enchanting 1, and a power focus 4 can actually be a surprisingly good enchanter even though they're primarily a summoner/spellcasting - 11 dice to enchant average potency 3.67, average successes on the roll 3.22
I also am annoyed that the economics of buying preparations make very little sense. Consider 3 aspected magicians trying to make it as wage mages - Alice (Enchanter, working on preparations), Bob (Sorcerer), Carol (Enchanter, working on focuses). All have magic 4, and achieve 4 successes on a magic+skill roll
Alice spends 8 hours a day, 20 days a month making preparations. The relation of the prices, drain values, and number of reagents needed written in Street Wyrd is inconsistent, but generally around 50 Nuyen/hour, so she makes 400 nuyen/day, 8,000 Nuyen/month. Given her preparations need to be completed and purchased around the same time, she probably needs someone managing commissions - assume they're living a low lifestyle. Adding in taxes from both of these people being SINners, she can make medium lifestyle with 300 Nuyen/month leftover for sundry expenses. Pretty good for someone living in Shadowrun, but not great for someone with a rare and valuable talent, working unpredictable hours.
Bob spends 4 hours a day, 20 days a month driving to different peoples houses and casting spells for them. Casting spells is instantaneous for him, but travel still takes time, so he casts 8 spells over the course of his day. He charges an average of 200 nuyen/spell - lower than most preparations since they can't be stored at all, though he does have slightly better results (4 successes, compared to the 2.67 average of a potency 4 preparation). He makes 1,600 nuyen/day, or 32,000/month. With the same low lifestyle secretary, he has enough for a high lifestyle, with 18,800 leftover per month. Enough to support a second person on a high lifestyle (perhaps a spouse, or hiring a bodyguard for all that driving around). A much, much nicer lifestyle than Alice, working half the hours.
Carol spends 8 hours a day crafting focuses, 5 days a month. She can craft 1 Force 1 focus each day. This costs her 2-6 reagent (100-300 Nuyen), and 1 karma (I'm counting this as 2,000 Nuyen, based on character creation). She makes 3.000-18.0000 per day, reduced to 900-15700 by operating costs. Assuming she makes exclusively weapon focuses, she makes 4850 nuyen/day, or 24250 Nuyen/month. Given that focuses last for a while she has much less need of commissions, but if she isn't working on commission she needs either a secure facility to keep these in, or to immediately sell them to a corporation with such a facility, either of which will take their cut. As long as it's not above 50%, she can make a high lifestyle, with SINner taxes, working 5 days a month. Probably with thousands of Nuyen to spare. If she works exclusively with power focuses, and works 10 days/month, she makes 157,000/month, enough for a luxury lifestyle, and 47,000 Nuyen to spare, maybe to hire 4 elite mercenaries as bodyguards at high lifestyle. (She needs a 4,500 Nuyen focus formula, and a 500 Nuyen lodge, but these can be paid off by working a day or two extra whenever she expands her workshop or adds a new product to her line). In any case, her work is maximally convenient - she picks when her shift is, and works then. The focus isn't going to go away just because she made it a few hours (or days, or months) before the customer wanted it.
Mostly unrelated - isn't it weird that the price of making a Focus is so unrelated to how valuable the kind of Focus is? I know, it's Shadowrun, not talismonger-simulator, but it bugs me.
On the other side of the sale, preparations are really expensive:
Why buy a 450 Nuyen arcane taser that will break in a day and can only be used once when you could buy a 340 Nuyen actual taser, a 100 Nuyen stun grenade, or hire someone for 5000 Nuyen a month (167 nuyen/day, though you probably need to hire 2 people to give them some time off) to follow you around shooting your 340 Nuyen taser at people, in case you're a bad shot?
Why buy a 550 Nuyen instant garden when you can hire a sorcerer to come over to your house and cast it for almost certainly less than 550 Nuyen for a few seconds work+travel time? Or hire a mundane gardener, or buy an already-grown plant?